was something like a figure eight with the ovals offset from
one another. The legs were cut from ABS plastic using a
Lasercamm, so their first incarnation lacked the friction to
be really useful.

Instead of propelling the snake forward, the slippery
legs would merely cause the bot to undulate on the floor as
if it were performing an aptly named dance move. To
improve the friction on the wheels and their ability to
transfer force in a useful way, we coated the legs in tool dip
(the rubber often found on the handles of tools to improve
grip). The tool dip added some much-needed friction,
allowing the legs to help lift the snake over high obstacles.

The effectiveness of the modular snake design was put
to the test in the most exciting way possible — a
competition. The goal of the search and rescue competition
held in conjunction with the 2004 AAAI conference was to
replicate the kinds of tasks that a robot might assist with in
a real world urban search and rescue situation. The driver
of the robot was situated in a tent on the other side of the
room, only able to see what the robot saw. The robot itself
would trek through a simulated disaster area replete with
debris and mannequins that represented the victims that
might be trapped during a disaster, and was tasked with
mapping the disaster area, finding the victims, and
assessing their condition.

To that end, PARC's modular snake was equipped with
cameras and carbon dioxide sensors, and it was deployed in
conjunction with a companion robot that distributed
waypoints throughout the arena that allowed the bot to
map the area. The modular design and RHex-like wheels
worked well in practice — the snake slithered through the
arena and tackled the uneven terrain effectively. In fact, the
snake did well enough to secure the PARC team third place
in the competition.

RHex-less Leg Syndrome

While the PARC modular snake used RHex-like legs in a way similar to the biologically
inspired hexapod, our previous experience was
not really RHex motion in the true sense. While
the ABS legs undoubtedly had some flex in
them, the bot was not using the spring
potentials to save the energy from each step.
The mechanical advantages from the modular
snake's legs came from a simpler mechanical
principle: the lever arm.

Wheels are not the best suited to
surmounting tall obstacles like curbs (or
calculus textbooks). When a wheel meets a
curb, it needs to generate a lot of friction to
be able to lift the robot vertically. A lever arm,
We thought we were well on our way to some fancy
footwork with our VEX robot in particular. When we were
first building the four wheeled driving base, we strived to
create a drive train with all of the makings of a champion
walker. Specifically, we focused on creating a drive train
with a lot of torque. While a lever arm (or perhaps lever
leg) might be more efficient at transferring force than a
wheel, it also acts as a multiplier for the amount of torque
required to move it.

The magnitude of the forces at work also urged us to
take precautionary measures to ensure the safety of the
servo motors and their internal gearboxes. Thankfully, more
torque and motor protection could be taken care of by one
design feature: a gear train with a gear ratio that trades off
speed for torque. The gear ratio — using a small input gear
and a large output gear — increased the torque of the drive
train, and the gears acted as a line of defense for the
servos. If something was going to get stripped out by the
forces at work, we would rather it be easily replaceable
external gears than a servo. Each wheel on the VEX robot
would also be individually driven, which would give us the
most control over how we wanted to align the legs relative
to one another.

One final design consideration was that we wanted the
shafts in the gear train supported on both sides, but we
needed the legs to spin freely. The trickiness of this
conundrum was likely a result of the limited number of VEX
parts we were working with, but it did require us to design
several iterations of the drive train before figuring out one